SANSUI HRJS21 – 21 Inch Semi flat CRT TV - FROM MY SERVICE TABLE TODAY

From My Service Table Today - SANSUI HRJS21 – 21 Inch Semi flat CRT TV - Dead Condition

Set got for repair Today; in dead condition.  The party said; the set has gone dead while working.  They have noted a variation in brightness level, before the set has gone dead.

  Something wrong to its horizontal scan output section circuit; I assumed, because, the set was working fine, and has gone dead condition; the party said so.  I asked the party; whether they heard any unusual noise from the set, before it gone dead.  They have informed me, that there was no such abnormal noise.  OK.  That’s all the details I could collect from the party.

  Just see; the party was seeing this set.  It went dead suddenly, without any reason; without any unusual noise from it.  If there was a noise did they heard, it might be of the blown up protection fuse inside the TV,  No noise heard, means, no such fault could be with this set.

  I opened the set.  Checked the AC fuse.  OK.  Means the party was right to their information.  Without blowing the AC mains fuse, here the set has gone dead.

I plugged the set to AC mains and switched it ON.  Measured the voltage across the main smoothing capacitor.  Showed  about 295VDC.  OK.  The rectifier section is working properly, but the SMPS section circuit does not.

Something short circuit to the DC voltage output of the SMPS.

To most SMPS circuit used today, have short circuit protection.  If any short circuit is detected by the main system control IC; to any of the DC output section of the SMPS, it will send a ‘Stop’ signal to SMPS controller, to defeat it.  That is happening here too.

Just see; the party has informed me that, they were seeing the program, while the set went dead.  I have this in my mind. 

One possibility to this fault; according to my service experiences, since 45 years; is the short circuited junction of the HOT [Horizontal Output Transistor] (Collector to Emitter).  I did measure the DC resistance across the 110VDC [Output of SMPS] supply line.  Shows a resistance less than 10 ohms. VERY LOW INDEED. A short circuit is detected.  De-soldered out the Horizontal output transistor; and measured the DC resistance again.  No short detected. More than 800 Ohms.

Yes.  The HOT was damaged.  Collector- to - Emitter short. 2SD2499.

Fault confirmed.

But, there should be some reason for the failure of HOT.  What might it be? Sometimes nothing.  Sometimes may have.  But, before replace this transistor, I should check for the reason; otherwise, the transistor I will replace too will be get shorted.

There are mainly two reasons for this fault.

Short circuited winding at the deflection yoke [horizontal scan section]

 The free-wheeling capacitor (the capacitor connected to the collector of the horizontal output transistor to Ground- usually will have a  value of 2200 to 9500Pf 2000VDC.)

If this capacitor has open circuit, means failed or any of its 2 solder terminals have any fault; loose soldering or so; the Horizontal Output Transistor will blow out, as soon as you switch ON the set.  You won’t get any time to measure any voltage abnormality.  The fault will occur within in a microsecond.  There is no doubt about it.

Although I've verified the fault, by replacing the HOT, the set worked fine.  But the it has lack of focus.

Adjusting the focus control at the horizontal output transformer didn't help much.

Further checking revealed that, the focus pin connection [At the CRT connector socket] is damaged.  Corroded.  Replaced  the CRT connector socket, at the CRT interface card. Adjusted both screen and focus voltage.

Checked OK.

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